The present invention relates generally to the lubrication of porous substrates, such as composite bearing members, and more particularly to coatings for sealing the surface of the substrate material for improved low friction response with conventional lubricants. The invention also relates to methods for coating composites, such as carbon-carbon composites, for improved tribological properties.
Friction and wear are fundamental considerations in the design and operation of mechanical systems, especially in advanced aerospace applications that demand high performance, reliability and long service life. State-of-the-art systems operate at higher speeds, under heavier loads and at higher environmental temperatures and extreme temperature gradients. Performance of conventional materials under operating conditions demanded by newer systems is typically marginal. Because of their unique and tailorable mechanical and physical properties, composites are a highly desirable choice for contacting, rubbing surfaces (tribo-components) operating under such extreme conditions.
The standard low cost, composite cage used today is made from a cotton fiber, phenolic matrix material (cotton-phenolic). This material is used in several large volume applications including machine tool spindle bearings. Additionally, U.S. Pat. No. 5,752,773 by Rosado et al. discloses a high performance composite cage made from carbon fibers in a carbon matrix (carbon-carbon) for gas turbine engines. This material has substantially higher temperature capability and better mechanical and thermal properties than cotton-phenolic. U.S. Pat. No. 5,752,773 is hereby incorporated by reference.
More recently, carbon fibers in phenolic matrix (carbon-phenolic) have been investigated as an intermediate cage material to replace cotton-phenolic in certain high-speed applications. Carbon-phenolic has substantially better thermal and mechanical properties than cotton-phenolic but does not have the high temperature endurance of carbon-carbon. So, each one of these materials fills a different niche: cotton-phenolic for low cost and moderate speed, carbon-phenolic for intermediate cost and high speed, and carbon-carbon for high temperature and high-speed conditions. However, all of these materials, as well as any porous composite material, has a relatively high coefficient of friction (COF), i.e. a COF greater than 0.15 is typical.
One example where these new composites will be beneficial is high-speed machine tool spindle bearings. Machine tool spindle bearings typically have cotton-phenolic cages and are lubricated with oil-mist or liquid lubrication. Thermal management and dimensional stability in the machine tool spindles is critical to maintain close dimensional tolerances on machine parts.
In another example, rolling element bearings for the next generation of gas turbine engines will operate at higher speed and load, and in higher temperature environments for improved engine performance. Thermal management of the bearing systems is critical to maintain adequate bearing life for safety and cost considerations.
Also, rolling element bearings in control moment gyroscopes and momentum flywheels for satellites typically use cotton-phenolic cages saturated with liquid lubricant. Control momentum gyroscopes and momentum flywheels can be made smaller, with lighter weight, if the rotor spins at higher speeds. Thermal management in lightly lubricated bearings is one of the primary challenges in making higher speed control moment gyroscopes and momentum flywheels.
Due to low density, high specific strength and high thermal conductivity, carbon-carbon (C—C) composites are very attractive for bearing cages used in high-speed applications or where marginal lubrication exists. Generally, liquid lubrication is used to produce the desirable low friction for the bearing cage applications. Normally with solid substrates, a friction coefficient of about 0.03 to 0.10 will be obtained if a liquid lubrication is applied between the substrates. The low friction is a result of a liquid film that forms to separate the two bodies in motion. This film is formed by hydrodynamic pressure due to motion. However, with C—C and similar composites, the porous nature of the composite does not permit the hydrodynamic film to form. The liquid gets absorbed into the bulk of the C—C composite instead of forming a lubricant film for lift off. Thus to obtain low friction, C—C composites require coatings to seal the pores to prevent the oil lubricant from seeping through thus allowing boundary lubrication. Additional coatings can further improve the response by providing lower friction at the asperity contacts of the seal coating.
Therefore, there is a need in the art for reducing the coefficient of friction of lubricated composites.